[PATCH 3/5] exec: Remove recursion from search_binary_handler

[Kees Cook]

On Tue, May 12, 2020 at 01:42:53PM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook at chromium.org> writes:
> > Should binfmt_misc do the install, or can the consuming binfmt do it?
> > i.e. when binfmt_elf sees bprm->execfd, does it perform the install
> > instead?
> 
> I am still thinking about this one, but here is where I am at.  At a
> practical level passing the file descriptor of the script to interpreter
> seems like something we should encourage in the long term.  It removes
> races and it is cheaper because then the interpreter does not have to
> turn around and open the script itself.

Yeah, this does sounds pretty good, though I have concerns about doing
it for a process that isn't expecting it. I've seen a lot of bad code
make assumptions about initial fd numbers. :(

> Strictly speaking binfmt_misc should not need to close the file
> descriptor in binfmt_misc because we have already unshared the files
> struct and reset_files_struct should handle restoring it.

If I get what you mean, I agree. The error case is fine.

> Calling fd_install in binfmt_misc still seems wrong, as that exposes
> the new file descriptor to user space with the old creds.

I haven't dug into the details here -- is there a real risk here? The
old creds are what opened the file originally for the exec. Are you
thinking about executable-but-not-readable files?

> It is possible although unlikely for userspace to find the file
> descriptor without consulting AT_EXECFD so just to be conservative I
> think we should install the file descriptor in begin_new_exec even if
> the next interpreter does not support AT_EXECFD.

I think universally installing the fd needs to be a distinct patch --
it's going to have a lot of consequences, IMO. We can certainly deal
with them, but I don't think it should be part of this clean-up series.

> I am still working on how to handle recursive binfmts but I suspect it
> is just a matter of having an array of struct files in struct
> linux_binprm.

If install is left if binfmt_misc, then the recursive problem goes away,
yes?

-- 
Kees Cook